Shotcreting is a process for pneumatically projecting mortar or concrete at high velocity onto a surface or a structure.
In the dry shotcrete process all ingredients except water and sometimes liquid accelerators are mixed in the dry state and the mix is conveyed by an air stream through hoses or pipes to a spray nozzle where water is added and the mix including water is thrown towards the structure to be covered.
In the wet shotcrete process a ready mixed concrete including water is transported through hoses or pipes to a spray nozzle where compressed air is applied to throw the mix towards the structure to be covered.
One of the disadvantages in the shotcrete process is the very high rebound losses which are experienced. When using ordinary concrete mixtures, the rebound losses both for the dry and the wet shotcrete processes may exceed 40% by weight of the total amount of concrete sprayed onto a surface. Further, by using ordinary concrete mixtures the layer thickness in one pass is limited to about five cm and lower.
For the wet shotcrete process the above drawbacks have been partly overcome by adding silica fume to the concrete batch. The addition of silica fume to the concrete batch greatly enhances the cohesiveness of the concrete and thus the rebound losses are reduced to below 10% and it is possible to apply a layer of 20 cm and more in one pass on vertical surfaces.
In the wet shotcrete process the silica fume is added to the concrete batch before it is pumped through the hoses to the spray nozzle.
It is also known to use silica fume in the dry shotcrete process. This is done by mixing dry silica fume with the other dry ingredients, cement and sand, whereafter the dry mixture is transported by air stream to the nozzle where water is added.
The above mentioned silica fume consists of extremely fine, spherical silica particles containing approximately 90% by weight of SiO.sub.2. The specific surface area is between 15 and 30 m.sup.2 /g and the particles have a diameter between 0.1 and 0.2 .mu.m.
This silica fume is obtained from the off-gases from electric furnaces for production of silicon or ferro-silicon. The silica fume can also be obtained by reduction of SiO.sub.2 to SiO-gas and reoxidation of SiO in air.
Due to the very small particle size and the high surface area the silica fume causes problems when added to the dry mix in the dry shotcrete process. Firstly, a dusting problem arises when adding the dry silica fume to the cement and sand. Further, it is very difficult to obtain a proper wetting of the silica fume particles in the nozzle. It is therefore necessary to use a nozzle with two water rings. Thus the dry mix is prewetted by adding water through a first water ring before the final water amount is supplied through the second water ring in the nozzle. Even with this prewetting, the silica fume particles are not wetted to a satisfactory extent and when the cement mixture reaches the surface to which it is sprayed the mixture is inhomogeneous. Thus the rebound losses are much higher than for wet shotcrete containing silica fume, but they are about half of the rebound losses of ordinary dry shotcrete mixture.